Manufacturing formulations involving aqueous solid dispersions often require to be processed at a high concentration of solids for one or more of the following reasons: to improve the costs/efficiency of transportation, to increase laydown or to reduce the drying load, as the removal of water is wasteful and expensive. The formulations also need to be stable over a wide range of shear, which extends from low to high shear rates. After mixing, aqueous dispersions of particulates, especially those containing high concentrations of solid, tend to form a ‘yield stress material’ as the shear stress is relaxed, the viscosities climbing dramatically with the decreasing shear such that, for example, gels may be deposited in zones of reduced shear. The high viscosity of the dispersions at low shear affects their dispersability and flowability, making the systems difficult to stir, pump, transport, coat or pour. Moreover their tendency to form a yield stress material or to gel in regions of low flow is often more of a problem, as gelation is often irreversible and consequently totally unacceptable for manufacturing processes that rely on systems to flow.
It is known in the art that an inorganic, organic or polymeric form of a base may be added to preparations of silica sols to minimize particle growth and stabilize the particulate dispersion. Thus U.S. Pat. No. 3,346,334 describes the use of a base, such as sodium hydroxide or ammonium hydroxide, to form relatively stable silica sols of particle size 2-3 nm and U.S. Pat. No. 3,714,064 discloses the use of an alkaline medium of pH 9-11 to prepare silica sols with particles less than 5 nm in size containing up to 25% silica. Steric stabilization has also been used in the case of very small particles to supplement ionic stabilization.
JP 54043937 and JP 54043938 describe a dispersant combination for silica sol coating compositions based on polyethylene glycol (t-dodecylthio)- or (t-tridecylthio)ethyl ether or polyethylene glycol sorbitan monoalkanoate, maleic acid or vinylpyrrolidone resin emulsion with, respectively, an alkaline solution of alginic acid or a solubilized styrene-maleic acid copolymer. These combinations are reported to provide excellent storability and pigment dispersibility under the conditions stated.
JP 61118130 discloses aqueous dispersants for fine silica particle (average diameter ˜0.1 μm) suspensions based on (meth)acrylic acid or α,β-unsaturated dicarboxylic acid homopolymers or copolymers with esters, acid-amides, and/or hydrophilic or hydrophobic monomers. An example is given where the addition of acrylic acid/methylacrylate copolymer as dispersant reduces the viscosity of a silica dispersion by over an order of magnitude and stabilizes the resulting dispersion for more than 90 days.
U.S. Pat. No. 3,360,954 describes the stabilization of sols of very small particles of silica by the use of a combination of an inorganic or organic base with a water-soluble nonaromatic polyhydroxy or hydroxyether compound, for example, polyvinyl alcohol.
JP 01156594 describes dispersing agents for powdered silica-containing paper coating agents comprising polyalkylenepolyamines and/or their derivatives and JP 04272888 discloses coating compositions containing silica for recording media, which also discloses dispersing agents based on polyalkylene-polyamine.
JP 2002095949 describes dispersants for dispersions of silica particles having high concentration and low viscosity, based on polymers containing >50 mol % water-soluble nonionic monomers, such as hydroxyalkyl (meth)acrylates. The application discloses an example showing that poly-(2-hydroxyethyl acrylate) is more effective at reducing the viscosity of a particular silica dispersion than a polymer based on poly(sodium acrylate).
JP 05064735 describes dispersants for dispersing inorganic fine powders, based on polymers obtained from monomer mixtures containing ≧20% ethylenically α,β-unsaturated carboxylic acids or their alkali metal, ammonium, or amine salts and solvents containing 1-150 parts alkylene glycols per 100 parts polymers. The polymers may also contain polyalkylene glycol mono(meth)-acrylate as comonomer. An example is disclosed based on the use of poly(acrylic acid) sodium salt as a dispersant for a dispersion of spherical silica particles (0.2 μm diameter) in ethylene glycol.